The invention first relates to a method and apparatus for management of individual cells within an ultracapacitor-based system.
The invention relates to a method and apparatus for management of individual cells within an ultracapacitor-based system.
Ultracapacitors are different from batteries in the following ways:                Batteries have a relatively flat discharge curve, the discharge curve for an ultracapacitor is capacitive in nature and follows the E=0.5CV2 where E is energy (in joules), C is capacitance (in farads), and V is voltage (in volts).        Ultracapacitors do not exhibit the “coup-de-fouet” (a sudden drop in voltage at the beginning of discharge of a lead-acid battery), so that the equivalent series resistance (ESR) is easily determined by delta V/delta I. This is less true for a battery.        Ultracapacitors are particularly sensitive to overvoltage and undervoltage conditions. When ultracapacitor cells are placed in series, variations in capacity, leakage current, and internal resistance can cause some cells to be overcharged while others are undercharged. This can result in conditions that are detrimental to the operation of ultracapacitor cells. Therefore, ultracapacitors must be equalized through some external means.        Ultracapacitors cannot be equalized in the same way as lead-acid or other batteries, which are typically subjected to long periods of charging at low currents.        
Doing this with an ultracapacitor would cause excessive pressure to build up within a cell (in the case of a sealed design) and electrolyte loss (in the case of a vented design). Therefore, a special circuit is required to ensure that ultracapacitors are equalized.                Some ultracapacitors can be discharged to zero without incident, whereas batteries cannot. Therefore, the charging circuit used for ultracapacitors must be capable of operating over a wide voltage range.        
A high-voltage string requires thousands of ultracapacitor cells placed in series, all of which require equalization. Ideally, individual cells should be monitored for current status and health. The method and apparatus described here is a monitoring, leveling, and control circuit designed to monitor, equalize, and charge individual cells in series to maintain optimal health of the cells and the entire ultracapacitor string.
In physical construction, the ultracapacitor cells in a high-voltage string are physically packaged into “modules,” each consisting of a number (typically 10 to 30) of cells, for simplicity of assembly. The construction of the monitoring, leveling, and control circuit can also be simplified by dividing its functions into module-level functions (operating at each module) and top level functions (operating over the entire high-voltage string).
At the module level, a low-cost local controller (local to the module) is used to select, monitor and charge individual cells in a particular module. At the top level, a more sophisticated master controller is used to control the local controllers, directing their functions. The local controllers communicate to a master controller through numerous voltage isolation circuits.
Because of the series connection of ultracapacitor modules, there is not a common electrical connection from which all communication signals can be referenced. The use of optical voltage isolation allows the establishment of a common reference on the master controller side of the isolator.
The communication between local controllers and master controllers is accomplished through the use of a communications protocol; in this case, I2C was used, although other protocols such as RS-232 may also be used. The communications protocol and data bus circuit must be capable of an adequate rate of data transfer, individual addressing of modules and/or cells, and communication to the local controller.
The local controller commands a charge and monitoring circuit, consisting of a non-isolated dc/dc converter with several advanced features. Among the more advanced features is the ability to change key voltage source parameters on command from local controller, without significantly interrupting operation of the circuit. For example, the switching frequency of the circuit can be adjusted for the use of advanced diagnostic techniques; this operation can be conducted from a remote location without taking the device off-line.
Another key advantage to the use of a local controller is immunity to noise. Previous inventions arrange the circuit so that the voltage level is brought back to the master controller, where it is digitized. This is acceptable for small systems, but is prohibitive in larger systems with thousands of cells. In this invention, the local controller performs all signal processing locally at the module, and sends data digitally to the master computer for post-processing.
To optimize operation and health of the ultracapacitor system, it is advantageous to charge and equalize individual cells as quickly as possible. Previous inventions, such as that described in U.S. Pat. No. 6,983,212, is limited to relatively low currents by the resistance of the circuit board traces. The invention described herein is capable of charging individual cells at a rate as high as 10 A, by using control algorithms such that the resistance of the current path does not limit the charge rate.
The invention described herein charges the ultracapacitor from a voltage source rather than the current source, which is more typically used. The voltage source configuration is safer than a current source, since it limits the voltage of the cell to the maximum setpoint, limiting the degree of overcharge to which it is subjected.
The invention described herein uses a novel approach to setting the current setpoint during charging. The voltage source configuration requires special circuitry to convert the voltage to a current; this is classically accomplished through the use of a resistor or transistor. These approaches cause inefficiency due to high power dissipation in the circuit. The current invention uses a interactive setpoint approach that monitors back current at the charge and monitoring circuit, sets the output voltage of the circuit to a level slightly higher than that of the cell, and measures the effect on the cell. If the resultant current is too high, output voltage can be increased; if the current is too low, output voltage can be decreased.
Similar devices described in the prior art, such as that described in U.S. Pat. No. 6,983,212 (incorporated herein by reference), require the use of 2N relays, where N is the total number of ultracapacitor cells. The present invention requires only N+1 relays, enabling significant cost savings and simplifying operation.